States either develop their own criteria or implement federal criteria for evaluating water quality. These criteria must accurately reflect the latest scientific knowledge. They are based solely on data and scientific judgments about pollutant concentrations and their effects on the environment, aquatic life, and human health. Montana water quality criteria include both numeric and narrative criteria. Water quality criteria for each use class are detailed in the Administrative Rules of Montana.

Numeric Criteria

Most of Montana’s water quality criteria are numeric; that is, the criteria define precise, measurable concentrations of pollutants that if exceeded would harm the use. Montana’s numeric water quality criteria are published in Circular DEQ-7 and Circular DEQ-12A.

Montana also has Nutrient Standards Variances which are published in Circular DEQ-12B.

Narrative Criteria

Some pollutants have narrative water quality criteria, which are statements (instead of specific quantities) that describe the desired water quality condition in terms of allowable ranges and maximums (e.g., water pH and temperature) or in terms of specific variation from natural conditions (e.g., water turbidity and color). Each use class defined in the rule has narrative criteria, and some narratives define an allowable change from naturally-occurring conditions. Naturally-occurring conditions are determined by reviewing historical data for a waterbody, if available, or by comparing conditions with a reference waterbody, an unaltered or otherwise in its most natural condition.

Montana’s water quality standards and details of how water quality standards are implemented in point-source discharge permitting are contained in Montana Administrative Rules. All current state water rules and Montana water quality standards are available on the Water Quality Regulations and Forms page.

High water quality (water quality better than the applicable water quality standard) must be maintained unless a lowering of water quality is necessary to accommodate important economic and social development.

Exceptional characteristics of specific waters designated as outstanding, very sensitive, or unique resources -- called outstanding resource value water -- must be maintained and protected.

The Federal Clean Water Act requires that states review their water quality standards every three years and solicit public input on the need for revising or adding to state standards. The review helps set priorities and a work plan for water quality standards development and rulemaking. DEQ completed the 2017 triennial standards review in May 2017. Documentation of the 2017 triennial standards review is provided below.

About Us

The Montana Department of Environmental Quality is charged with protecting a clean and healthy environment as guaranteed to our citizens by our State Constitution. Our ultimate goal is to protect public health and to maintain Montana's
high quality of life for current and future generations.